Theme Ideas For My 18t Birthday Party

My 18th birthday is coming up and i dont have any ideas for a theme. I want it to be something no one will forget and be totally fun. Everyone always does costume, i dont want that, i want something original and something i can do at my house and in the back yard. I was wondering about doing something like "one day in paris" or "one day in London" but i dont know how i could decorate the house or the backyard with that kind of theme. It could be somewhat costume where they match the theme but i dont want it to be like halloween type costume. I want it to be special, something i`ll always remember. Thank you.